Virtual Frisch-grid detectors represent a class of single-carrier devices for which fast-moving electrons are used to measure energies deposited by X-rays, gamma-rays, ionizing particles, and the like. A main drawback of single carrier devices is that the output signals are influenced by the stationary holes whose effects depend on the locations of interaction points inside the device. Such behavior of the output signals, caused by uncollected holes, is commonly called the “induction effect.” This effect inhibits spectral resolution of single-carrier detectors unless special measures are implemented to neutralize the holes. Two techniques directed to minimizing the induction effect are: (1) subtracting the fraction of the charge signal contributed by the stationary holes and (2) electrostatic shielding of the stationary holes.
In the first technique, the induced charge contributed by the holes is measured and subtracted from the total output signal. The holes-induced signal can be measured with one or several electrodes located adjacent to the collecting electrode. The electrodes can be composed of pixels or strips and, depending on the locations of interaction points, the same electrode can be used for measuring collected or induced-only charges. Coplanar-grid devices proposed for Cadmium Zinc Telluride (CdZnTe or CZT) and fluid Xenon (Xe) detectors are special cases for implementing this technique.
For the second technique, there are two types of devices that employ the electrostatic shielding: Frisch-grid and virtual Frisch-grid detectors. In a classic Frisch-grid ionization chamber, a metal grid (or mesh) is used for electrostatic shielding of the collecting electrode (or several collecting electrodes) from the positive ions. In a virtual Frisch-ring detector, the shielding electrode (or several electrodes) is placed around the sides of the device's sensitive volume but they produce essentially the same shielding effect as if a real Frisch grid were placed inside the detector.
In the past, several designs of virtual Frisch-grid detectors were proposed for CdZnTe semiconductor: CAPture™, hemispherical, Frisch-ring, and pixel detectors. (CAPture™ is a trademark of eV Products, Inc. of Saxonburg, Pa.). CAPture™-type and hemispherical type detectors are generally produced by extending the cathode electrode up the sides of the detector crystal body. In both devices, the extended electrodes are in physical contact with the semiconductor surfaces. As a result, a wide area of the bare detector crystal body surface surrounding the anode is required to keep the surface leakage current below an acceptable level. While these convention devices typically achieve acceptable results for low energy gamma-rays, which normally interact close to the cathode, these conventional devices typically do not perform as well for higher energy gamma-rays that interact closer to the anode. Since the area near the anode is not properly shielded, convention Frisch-grid detectors usually exhibit low energy tails for peaks in the pulse height spectra.
FIG. 1A depicts a perspective view of a conventional virtual Frisch-grid detector 100 and FIG. 1B depicts a cross-sectional view along a longitudinal axis 102 of the conventional virtual Frisch-grid detector 100. In this example, the conventional virtual Frisch-grid detector 100 is a CAPture™-type detector that has an extended cathode 104 formed at one end 106 of a detector crystal 108. An unshielded and uninsulated area 110 lies between the cathode's edge 112 and an anode 114 formed at the other end 116 of the detector crystal 108; thereby leaving the detector crystal susceptible to electrostatic interference.
In conventional Frisch-ring detectors, the cathode is also extended along the longitudinal surface of the detector, but the extended portion of the cathode is physically separated from the longitudinal surface by a thin layer of insulating material. The non -contacting electrode, also known as the non-contacting Frisch ring, is the main feature that makes Frisch-ring detectors different from CAPture™ and hemispherical-type devices. The non-contacting electrode is placed along the longitudinal surface of the crystal and is physically connected to the cathode. A common high-voltage bias (with respect to the anode) is applied to the cathode and the non-contacting electrode. The output signal is read out from the anode. Thus, a gap (typically 2-3 mm) of insulating material is generally left between the Frisch-ring and the anode's contact to prevent high leakage current or even possible discharge in the area between these two electrodes. As a result, an electrostatically unshielded area of the surface exists near the anode, which results in a low energy tailing effect discussed above.
FIG. 2A depicts a perspective view of a conventional Frisch-ring detector 200 and FIG. 2B depicts a cross-sectional view along the longitudinal axis 202 of the conventional Frisch-ring detector 200. In this example, the longitudinal surface 204 of the detector crystal 206 is insulated with an insulator 208 leaving only the ends 210 and 212 available for making electrical contact. The cathode 214 is extended along the longitudinal length of, but electrically isolated from, the detector crystal 206. However, an unshielded area 216 between the edge 218 of the cathode 214 and an anode 220 is generally required to prevent leakage current or even electrical discharge between the cathode 214 and the anode 220.
Conventional virtual Frisch-grid detectors generally have three common effects that result in degradation of their performance. The first effect that degrades their performance is the presence of an unshielded area near the anode. The second effect that degrades their performance is that the electric field inside the detector (drift field) does not generally direct electrons, liberated by the incident photons or particles, toward the anode (defocusing field). The third effect that degrades their performance is the two-terminal nature of devices in which the cathode signal cannot provide the particle's interaction depth information to correct the electron losses due to trapping.